1. Field of the Invention
The present invention relates to mutant bacterial strains of the genus Sphingomonas that are deficient in production of an internal storage polymer, polyhydroxybutyrate (“PHB”) due to a null mutation, but produce normal quality of the capsular polysaccharides commonly referred to as sphingans. The present invention also relates to a method of clarifying the sphingans produced by a mutant strain of Sphingomonas that is deficient in the production of PHB. The present invention further relates to food or industrial products comprising PHB-deficient and/or clarified sphingans.
2. Discussion of the Related Art
Sphingans are capsular polysaccharides secreted by bacteria of the genus Sphinqomonas. Sphingans are structurally related, but not identical. Common members of the genus Sphingomonas and the sphingans they produce include Sphingomonas elodea, ATCC 31461, which produces gellan (S-60); Sphingomonas sp. ATCC 31555, which produces welan (S-130); Sphingomonas sp. ATCC 31961, which produces rhamsan (S-194); Sphingomonas sp. ATCC 53159, which produces diutan (S-657); Sphingomonas sp. ATCC 31554, which produces an as yet unnamed polysaccharide (S-88); Sphingomonas sp. ATCC 31853, which produces an as yet unnamed polysaccharide (S-198); Sphingomonas sp. ATCC 21423, which produces an as yet unnamed polysaccharide (S-7); Sphingomonas sp. ATCC 53272, which produces an as yet unnamed polysaccharide (NW-11); Sphingomonas sp. FERM-BP2015 (previously Alcaligenes latus B-16), which produces alcalan (Biopolymer B-16) and the like. A description of the Sphingomonads and the polysaccharides they produce can be found in U.S. Pat. Nos. 4,377,636; 4,326,053; 4,326,052 and 4,385,123 (for ATCC 31461 and its S-60 polysaccharide); in U.S. Pat. No. 4,342,866 (for ATCC 31555 and S-130); in U.S. Pat. No. 4,401,760 (for ATCC 31961 and S-194); in U.S. Pat. No. 5,175,278 (for ATCC 53159 and S-657); in U.S. Pat. Nos. 4,331,440 and 4,535,153 (for ATCC 31554 and S-88); in U.S. Pat. No. 4,529,797 (for ATCC 31853 and S-198); in U.S. Pat. No. 3,960,832 (for ATCC 21423 and S-7); in U.S. Pat. No. 4,874,044 (for ATCC 53272 and NW-11); in U.S. Pat. No. 5,175,279 (for FERM BP-2015 and B-16), all of which are incorporated by reference herein.
Sphingan polysaccharides are structurally related by the primary structure of their backbone, which comprises the sugars D-glucose, D-glucuronic acid, and L-rhamnose (or L-mannose). For example, the primary structure of gellan, S-60, comprises the sugars D-glucose, D-glucuronic acid and L-rhamnose in a 2:1:1 molar ratio, which are linked together to form a tetrasaccharide repeat unit in the following order: glucose, glucuronic acid, glucose, rhamnose. In the native form, gellan is modified by acetyl and glyceryl substituents on the same glucose residue. On average, gellan has one glycerate substituent per tetrasaccharide repeat unit and one acetate substituent per every two tetrasaccharide repeat units. The primary structure of another sphingan, diutan, S-657, differs from gellan in that it has an additional disaccharide side chain of L-rhamnose attached to one glucose residue, thus forming a hexapolysaccharide repeat unit. S-657 contains acetyl groups at position 2 and/or position 6 of the other glucose residue.
Sphingan polysaccharides, which are also referred to as gums, are primarily used to thicken or gel aqueous solutions and are frequently classified into two groups: thickeners and gelling agents. Typical thickeners include starches, guar gum, carboxymethylcellulose, alginate, methylcellulose, xanthan, gum karaya and gum tragacanth. Common gelling agents include gellan, gelatin, starch, alginate, pectin, carrageenan, agar and methylcellulose.
Gelling agents are used in the food industry in a variety of applications, including confectionary jellies, jams, dessert gels, icings, dairy products, beverages and the like. Additionally, gelling agents may be used as components of microbiological media. Gelling agents differ in the conditions under which they may be used and in the texture of the gels they form. These distinctive properties of gels have led to the exclusive use of certain gelling agents in particular products (e.g., starch in confectionary jellies; gelatin in dessert gels; agar in icings; and alginate in pimento strips).
Despite the use of certain gelling agents in particular products, disadvantages exist for conventional food formulations. For example, gelatin, which is frequently used in dessert gel formulations, is animal-sourced, requires refrigeration to set and is limited in application due to its instability under heat. Carrageenan, carrageenan and locust bean gum blends, and pectin, which are frequently used in dessert gel, confectionery and jam/jelly formulations, are generally limited to formulations that are brittle and inelastic in texture, suffer from poor storage stability and may be geographically restricted from use in some countries, such as Japan. Starch, which is frequently used in confection formulations, provides poor clarity and poor flavor release. Consequently, it would be desirable to develop a gelling agent for use in food formulations that is free from the problems associated with conventional gelling agents.
One particularly useful gelling agent is gellan (S-60), which is a capsular polysaccharide produced by the bacterium Sphingomonas elodea, ATCC 31461. Commercially, the gum is formed by inoculating a fermentation medium under aerobic conditions with Sphingomonas elodea bacteria. The fermentation medium contains a carbon source, phosphate, organic and inorganic nitrogen sources and appropriate trace elements. The fermentation is conducted under sterile conditions with strict control of aeration, agitation, temperature and pH. Upon completion of the fermentation, the viscous broth is pasteurized to kill viable cells prior to recovery of the gum. However, the optimal fermentation conditions for producing gellan also promote production of the internal storage polymer, polyhydroxybutyrate (“PHB”), which interferes with the ultimate clarification and recovery of gellan. During fermentation, PHB synthesis and gellan synthesis compete for the available carbon source, and PHB synthesis may compete with gellan synthesis.
Gellan displays different characteristics depending upon the method of recovery from the fermentation broth. Direct recovery from the fermentation broth yields gellan in its native or high-acyl form, which is modified by S. elodea with acetyl and glyceryl substituents on one glucose residue. Isolation of gellan in this native or high-acyl form yields a soft, flexible, elastic gel. Gellan may be deacylated by treatment with hot alkali, thereby providing gellan in its low acyl form. Isolation of gellan in this deacylated form yields a hard, firm, brittle gel, which has limited commercial applications. Blends of native and deacylated gellan produce gels of intermediate texture.
Certain applications require clear gellan. Currently, however, only deacylated gellan can be clarified. During the deacylation process, gellan is treated with alkali at high temperature, which removes the acyl substituents from the gellan and lyses the S. elodea cells. Solids and cell debris are then removed by filtration yielding a clear, non-acylated gellan. To date it has not been possible to clarify gellan in its native or high-acyl form via filtration due to the high set temperature (the temperature at which a gum forms a gel upon cooling) required and the capsular nature of the organism, which does not allow facile separation of gellan from the S. elodea cells. For applications requiring native gellan, S. elodea cells may be lysed chemically or enzymatically; however, the remaining PHB will be present in the final product and renders the resulting solutions turbid, rather than clear.
In addition to the use of gellan as a gelling agent, other sphingan polysaccharides have also found useful commercial application. The S-657 polysaccharide imparts significant pseudoplasticity to polar solvents such as water, such that S-657 can act as a rheological modifier that is capable of particle suspension, friction reduction, emulsion and foam stabilization, filter cake disposition and filtration control. Consequently, S-657 has found industrial utility as a rheological modifier in a variety of cementitious systems, as disclosed in U.S. Pat. No. 6,110,271, which is incorporated herein by reference.
In addition to impairing clarity, the PHB found in sphingans affects the rheological properties of their gums. In particular, the PHB in S-657 gum affects the ability of the polysaccharide to modify rheology in porous medial flow environments such as oil fields, wherein rheology plays a significant role in well-bore drilling, completion and workover fluids. In addition, PHB residue in S-657 may cause damage during reservoir formation and may reduce the productivity of wells. The presence of PHB furthermore limits the applicability of S-657 gum in household and personal care products, in which appearance is critical to consumer acceptance.
Accordingly, attempts have been made to eliminate PHB production in sphingans. One way to alleviate the problem of interfering PHB production in Sphingomonas species has been to chemically induce a random mutation into a strain that inhibits production of PHB, such as described in U.S. Pat. No. 5,300,429, which discloses LPG-2, a mutant strain of Sphingomonas elodea that inhibits the production of PHB, but remains capable of producing gellan. Sphingomonas elodea was formerly known as Pseudomonas elodea and refers to the same organism. The LPG-2 strain is on deposit with the American Type Culture Collection and designated ATCC 53967. While the LPG-2 strain produces gellan, its quality is inconsistent, presumably due to the additional mutation(s) which occur with chemical mutagenesis.
Genetic engineering is a more selective mutagenesis approach for generating null mutant strains of Sphingomonas deficient for production of PHB. Genetic engineering permits selective mutation or deletion of a gene within the PHB synthesis pathway, which in turn permits complete inhibition of PHB production without affecting the quality of gum production.
Consequently, it would be highly desirable to develop mutant strains of Sphingomonas that are deficient in their ability to synthesize PHB, while maximizing sphingan production and, concomitantly, mitigating the requisite effort to remove PHB from sphingans.